IP/UDP/RTP header fields may be generally classified into static, delta, dynamic, and inferred attributes. Static is a field having a fixed value in one end to end packet stream. This field corresponds to an IP address and port number. In addition, this field also corresponds to a field, such as a RTP or IP version field, having a well-known value. Delta is a field having a fixed value difference from a previous packet. This field corresponds to sequence number. Dynamic is a randomly changeable field. This field corresponds to checksum and ID of an IP packet. Inferred corresponds to a field which may be inferred through another header field, such as a length field. A context identifier (CID) concept is introduced for a general header compression scheme. When a compressor sends a packet having an uncompressed full header while a specific CID is added thereto first and sends subsequent packets while omitting header fields having static, dynamic, or inferred properties as the same CID, a decompressor recovers all RTP headers by adding the omitted field to the compression header received after the second packet with reference to initially stored header field information based on the CID. For the delta header, the compressor and the decompressor store most fields of the full headers and, when the compressor transports only a difference value from a previous packet, the decompressor corrects the difference value using a previously stored value to perform recovery.
Robust Header Compression (RoHC) is a standardized scheme for compressing headers, such as IP, UDP, RTP, and TCP. In a streaming application, the IP, UDP, and RTP each have an overhead of 40 bytes for IPv4 and 60 bytes for IPv6. For VoIP, this value is equivalent to 60% all transported data. Such a great overhead may cause a serious problem in a wireless system having a limited bandwidth. Through RoHC, the overhead of 40 bytes or 60 bytes is compressed to 1 or 3 bytes, and decompression is performed after being delivered to the decompressor.
In RoHC, header fields are classified into static, dynamic, and inferable. A compression state in the compressor may be defined as Initialization and Refresh (IR), First Order (FO), and Second Order (SO) and a decompression state in the decompressor is defined as No Context (NC), Static Context (SC), and Full Context (FC). The RoHC scheme is to start transport at a low compression rate and to maintain a state in which transport is performed at the possible highest compression rate. When the decompressor fails to perform context initialization or decompression, a state of the compressor returns to IR, which is the lowest compression step, and in this state the compressor transports full headers. Subsequently, at the FO step, the compressor omits the static field. Finally, at the SO step, all compressible fields are not transported. A state of the decompressor may be changed from NC, which is the lowest step, to the SC and FC steps. At the FC step, an optimal decompression operation is performed.
RoHC performs compression in a scheme in which total headers are transported at the beginning of transport and unchanged portions are omitted in the middle of transport. In a case in which this scheme is adopted by a broadcasting system, a broadcast receiver may not know when to receive an IP stream and a receiver which does not know all header information may not recognize a corresponding IP packet.